1. Field of the Invention
The present invention relates to a vacuum lift device.
2. Description of the Related Art
A known vacuum lift device comprises a vacuum operated type lift tube which is able to expand and contract in an upward and downward direction, and a lift tube expansion control valve controlling the level of vacuum in the lift tube by controlling the opening area of the atmospheric opening of the lift tube to cause the lift tube to expand and contract in an upward and downward direction (see Japanese Examined Utility Model Publication No. 62-25344). In this vacuum lift device, for example, an object to be lifted is initially sucked and held by the lower end portion of the lift tube, and then the object is lifted by contracting the lift tube by operating the lift tube expansion control valve. Then, the object thus lifted is maintained at a desired height by operating the lift tube expansion control valve. Then, the object is caused to move to another place in a state where the object is maintained at a desired height. Then, the object is lowered and placed there.
As mentioned above, a contraction control for lifting the object, a contraction and expansion stopping control for maintaining the object at a desired height and an expansion control for lowering the object are required for the lift tube. In a known device, these three controls are carried out by using only the lift tube expansion control valve.
In such a vacuum lift device, the lift tube expansion control valve is completely closed when the level of vacuum in the lift tube is to be made maximum, and the lift tube expansion control valve is fully open when the level of vacuum in the lift tube is to be made minimum. In this case, the quicker the level of vacuum can be lowered the larger the opening area of the atmospheric opening, obtained when the lift tube expansion control valve is fully open, is made. Accordingly, the opening area of the atmospheric opening, obtained when the lift tube expansion control valve is fully open, is normally made relatively large. Namely, to quickly expand and contract the lift tube, the control range of the opening area by the lift tube expansion control valve is normally made considerably wide, and therefore, if the lift tube expansion control valve is slightly displaced, the opening area of the atmospheric opening changes considerably.
However, to maintain the object at a desired height, the level of vacuum in the lift tube must be maintained at a constant level corresponding to the weight of the object. Namely, in this case, if the level of vacuum in the vacuum lift becomes slightly higher than this constant level, the lift tube contracts and, if the level of vacuum in the vacuum lift becomes slightly lower than this constant level, the lift tube expands. That is, to maintain the object at a desired height, the precise control of the opening area of the atmospheric opening is required.
However, in a known vacuum lift device, as mentioned above, if the lift tube expansion control valve is slightly displaced, the opening area of the atmospheric opening changes considerably, and thus it is difficult to carry out the precise control of the opening area of the atmospheric opening by the lift tube expansion control valve. Accordingly, a problem arises in that, if the opening area of the atmospheric opening is controlled by the lift tube expansion control valve so as to maintain the object at a desired height as in this known vacuum lift device, it is difficult to properly maintain the object at a desired height.